Itachi kills the Uchiha
Hence they were pushed to the corner of Konoha and were sidelined. This created a disarray among the Uchiha. They were not happy about it. Hence, they conspired a coup d’état.
Itachi Uchiha at this time was a budding shinobi, in fact he went on to make it to the ANBU Black Ops, the special assasination squad of Konoha. So, Fugaku taking advantage of this situation asked him to spy on the village.
But Danzo knew better, he asked the reverse of Itachi, he asked him to monitor the Uchiha clan’s activities and report back to him. It was a two battle already.
However, Itachi witnessing the horrors of war knew that a coup d’état would start another Ninja war was against it!Itachi more than anyone knew what a war meant. He had experienced it at a very young age.
It meant destruction of property, lives and disarray among people. He didn’t want something like that. He was a pacifist.